In the compressor portion of an aircraft gas turbine engine, atmospheric air is compressed to 10–25 times atmospheric pressure, and adiabatically heated to 800°–1250° F. in the process. This heated and compressed air is directed into a combustor, where it is mixed with fuel. The fuel is ignited, and the combustion process heats the gases to very high temperatures, in excess of 3000° F. These hot gases pass through the turbine, where rotating turbine wheels extract energy to drive the fan and compressor of the engine, and the exhaust system, where the gases supply thrust to propel the aircraft. To improve the efficiency of operation of the aircraft engine, combustion temperatures have been raised. Of course, as the combustion temperature is raised, steps must be taken to prevent thermal degradation of the combustors.
Combustors used in gas turbine engines, such as aircraft engines, typically are constructed of thin-walled materials. A conventional combustor is an assembly that is comprised of five individual pieces. The flame side of the combustor typically is coated with a thermal barrier coating which reduces the heat transfer rate to the liners and the dome. The frame for the combustor is the combustor dome. Inner and outer cowls, typically which are sheet metal parts, and inner and outer liners are bolted to the dome at its inside and outside diameters. The inner and outer liners typically are fabricated from individual machine forgings and welded together. The combustor dome is comprised of a single spectacle plate made of a die formed sheet metal part. Individual swirl cup packages are brazed to the spectacle plate. The swirl cup packages include the primary swirler with its retainer, a counter rotating secondary swirler, a venturi and a splash plate. Fuel is injected through fuel injector or fuel nozzle. The splash plate, like other components of the swirl cup package, is heated by the combustion of the fuel/air mixture occurring downstream from the splash plate and cooled by impingement of cooling air which is passed through holes in the combustor dome impinging on the back or upstream surface of the splash plate.
As temperatures of gas turbine engines have continued to increase, the combustion temperatures have become sufficiently high that even the best superalloy materials exhibit shortened lives due to thermal degradation. This is true even of the superalloys used for splash plates in high efficiency, advanced cycle turbine engines which are prone to failure by thermal degradation. As combustion temperatures have increased, the impingement cooling and thermal barrier coatings have been inadequate to provide sufficient cooling to maintain component life without thermal degradation. Various attempts have been made to improve the resistance to thermal degradation which have provided incremental improvements. These have included high temperature reflectors referred to as “Spray and Bake” coatings. These reflectors include platinum paints and platinum layers applied by chemical vapor deposition deposited over silicon dioxide (SiO2). These reflectors act by reflecting heat away from the splash plate rather than having the heat absorbed by the splash plate, conducted through the splash plate and then removed from the back side (or upstream side) of the splash plate by convection. Ideally, the heat is reflected back into the flow of combustion gases moving downstream into the turbine portion of the engine. However, these “Spray and Bake” coatings become ineffective as the temperatures approach 2150° F.
Reflectors such as multilayer dielectrics are available for use to improve the thermal capability of components in the hot gas flow path. Such dielectric mirrors are comprised of multiple layers of a high index and low index transparent solids, deposited at a thickness of about ¼ of the wavelength of the radiation to be reflected. However, the cost versus the benefit achieved by these mirrors negates their use for Commercial Engine Operations (CEO). What is needed is a cost effective coating that can act as a reflector to assist in cooling a thin splash plate by reflecting radiative heat back into the combustion gas stream. The coating must be sufficiently thin so as not to increase the weight of the component substantially, yet reduce the radiative heat absorbed by the splash plate so that the splash plate can operate over the expected life of the combustor without experienced deterioration due to thermal degradation that requires its replacement.